Industry is continually searching for new salts which can provide ionic conductivity when dissolved or dispersed in other materials. Such salts are especially useful when employed in combination with other materials to form electrolytes to conduct electrical charge in high energy density, lightweight, rechargeable power sources for use in automotive, industrial, and consumer markets, for example, electrochemical cells and devices such as batteries, fuel cells, capacitors, supercapacitors and electrochromic devices.
Many of these emerging power sources employ lithium-ion battery technology, which requires the use of an electrolyte consisting of conductive salt(s) dissolved or dispersed in matrix materials such as non-aqueous solvent(s) or polymer(s). This electrolyte acts as the medium through which ionic conduction can occur between electrodes, thus providing charge balance within the battery.
Of course, new electrolyte salts must exhibit specific chemical and physical properties to be useful in electrochemical cells and devices. Of primary importance, the salts must exhibit good ionic conductivity and should be thermally and electrochemically stable. Additionally, the salts must also exhibit good solubility at high concentration in common electrolyte solvents and/or polymers; they should exhibit inertness to other battery components (e.g., not cause corrosion of electrodes or current collectors); they should be relatively non-toxic; they should have acceptable environmental impact; and preferably they can be produced at an economically feasible price. In the case of secondary (i.e., rechargeable) batteries, the salts should exhibit good cycling behavior at room temperature and elevated temperature and should produce electrochemical cells that can be operated and maintained with minimal concerns for safety (e.g., explosions caused by thermal runaway).
There are currently only a small number electrolyte salts known to be suitable for use in lithium-ion batteries; all are lithium salts and all have identifiable drawbacks. The most common electrolyte salt is LiPF.sub.6, an inorganic salt which exhibits good conductivity and corrosion resistance, but is thermally and hydrolytically unstable, decomposing to liberate fluoride ion which is detrimental to cell performance. Other inorganic salts having potential use in lithium electrolytes include LiAsF.sub.6 (toxic), LiBF.sub.4 (relatively poor conductivity, thermally and hydrolytically unstable), and LiClO.sub.4 (thermally unstable, potentially explosive). There are also a number of organofluorine lithium salts known to be useful in battery electrolytes, but each of these salts has its own individual short-comings. Molecules like LiOSO.sub.2 CF.sub.3 and LiN(SO.sub.2 CF.sub.3).sub.2 are thermally very stable but can be corrosive to aluminum current collectors in high voltage batteries. LiC(SO.sub.2 CF.sub.3).sub.3 is prohibitively expensive for use in most commercial scale electrochemical cell applications.
There is a continuing need for new electrolyte salts which can perform at useful conductivity levels, show low corrosivity toward aluminum current collectors, are easily handled, and can be produced at a reasonable cost.